With increased requirements in recording density in recent years, magnetic recording media have been demanded that employ ferromagnetic powder that is finer in particle size and which have magnetic layers which are higher in surface smoothness. However, such finer ferromagnetic powder and smoother magnetic layer surfaces result in an increase in the coefficient of friction of the magnetic layer surfaces. As a consequence, it becomes difficult to improve running durability and running stability without adversely affecting wow and flutter characteristics.
For a conventional plural magnetic layer coating, sequential coating has been mainly employed. Typically, improvements in the S/N ratio and the output have been made by reducing the particle size of ferromagnetic powder contained in a second magnetic layer (an upper layer).
However, when the particle size of the ferromagnetic powder contained in the upper layer is reduced, the dropping off of powder from the magnetic layer (powder dropping) is liable to take place, which causes clogging and hinders running. Further, the .mu. value (coefficient of friction) of the magnetic layer is increased which tends to introduce a problem also for running stability.
Many headphone stereos have come to be used for audio cassettes. The cassette players for these mobile systems are minimum reproducers for reproduction capability only typically which are very convenient to transport or carry. However, the consumption power is reduced to permit miniaturization of the reproducers. As a result, the cassette players/reproducers are weaker in transfer power than conventional recorders for audio cassettes which, as a result, are susceptible to increase in wow and flyutter. Magnetic recording media having better running stability than conventional magnetic recording media have been desired to offset such disadvantages associated with the miniaturized players.
In this regard, it is effective to incorporate a large amount of carbon black in the second magnetic layer (the upper layer) to improve running stability. However, an increase in the amount of the particles contained in the upper layer introduces or aggravates the problem of powder dropping during running.
In order to avoid such a powder dropping problem, one approach has been to increase the amount of a binder contained in the upper layer, as described in JP-A-58-56228, JP-A-58-56230 and JP-A-54-48504 (the term "JP-A" as used herein means an "unexamined published Japanese patent application").
However, in the case of sequential coating, if the amount of the binder contained in the upper layer is increased, a clear and distinct interface is formed between the upper and lower layers which are largely different from each other in binder amount distribution. For this reason, internal strain is created and retained in the magnetic layers. As a result, the upper magnetic layer containing the binder in a larger amount contracts strongly.
The retaining of strain in the magnetic layers leads to easy separation of the upper magnetic layer in repeated running and aggravation of the problem of powder dropping. Further, the magnetic layers markedly curled or warped, which makes it impossible to ensure stable contact with a head.
Namely, for the audio tape obtained by the conventional sequential coating procedure in general, the second magnetic layer (the upper layer) is coated after completing the hardening of the first magnetic layer (the lower layer) has been completed. As a result, a clear and distinct interface also appears between the magnetic layers, which are usually largely different from each other in binder amount distribution, and internal strain is therefore developed in the magnetic layers. In extreme cases, the phenomenon that the second magnetic layer is separated from the first magnetic layer is observed, and abnormalities such as clogging sometimes occur during running. Even when the separation does not take place, the tape is markedly curled, which often makes it impossible to maintain suitable contact with the head.
In order to solve the problems associated with the sequential coating procedure described above, simultaneous plural-layer coating, or wet-on-wet coating, has recently been developed. For example, the kind of binders and the amount of functional groups contained in the binders are adjusted to ameliorate deterioration of running durability due to a reduction in the particles size of ferromagnetic powder contained in upper layers, as described, for example, in JP-A-63-146211, JP-A-63-146209, JP-A-63-146210, JP-A-63-261529 and JP-A-63-261530.
In such a wet-on-wet coating method, an embodiment, in which the amount of binders contained in upper and lower layers is specified, is disclosed in JP-A-2-101627. A magnetic recording medium described therein is designed for video cassettes. The medium comprises a non-magnetic support having thereon a first magnetic layer and a second magnetic layer in that order, the second magnetic layer being formed while the first magnetic layer is still in a wet state, and each of the first and second magnetic layers containing a vinyl chloride resin or a cellulose resin as a part of a binder, and a ratio ((A/C-1).times.100) of a total amount (A) of the binder contained in the first magnetic layer based on the amount of a ferromagnetic powder contained in the first magnetic layer to a total amount (C) of the binder contained in the second magnetic layer based on the amount of ferromagnetic powder contained in the second magnetic layer being .+-.5% to .+-.20%.
However, the magnetic recording medium designed for video cassettes are different from the magnetic recording media used for audio cassettes in design principle. It is not typically possible to interchange the former for the latter. Namely, for magnetic recording medium video cassettes, the problem of curling is not encountered, because the first magnetic layer is relatively thin. On the other hand, the media for video cassettes are not satisfactory in electromagnetic characteristics for recording of long wavelength such as typical to an audio tape, because the first magnetic layer is thin, and has the particular disadvantage that the output, the MOL, at 10 kHz decreases, as associated with the use of the same binder in the first layer and the second layer. In recent years, an improvement in quality of a high tone has been required in audio tapes, and a reduction in output at 10 kHz has been a serious problem.